In positron emission tomography (“PET”), a radioactive material is placed in the patient. In the process of radioactive decay, this material emits positrons. These positrons travel through the patient until they encounter electrons. When a positron and an electron meet, they annihilate each other. This results in emission of two gamma ray photons traveling in opposite directions. By detecting these gamma ray photons, one can infer the distribution of the radioactive material within the patient.
Certain materials, referred to as scintillating crystals, emit an isotropic spray of scintillation photons centered at a point at which a gamma ray interacts with the material. Some of these scintillation photons are emitted in a direction that takes them to a photodetector. Other scintillation photons, which are emitted in a direction away from any photodetector, nevertheless manage to reach a photodetector after being redirected by structures within the scintillating crystal. Yet other scintillation photons are absorbed and therefore never reach the photodetector at all.
To detect gamma ray photons, the patient is positioned within a ring of scintillating crystals. Photodetectors observing the crystals can then detect the scintillation photons and provide, to a processor, information on how many coincident gamma ray photon pairs were received in a particular interval and at what location those gamma ray photon pairs originated. The processor then processes such data arriving from all photodetectors to form an image showing the spatial distribution of radioactive material within the patient.
Each photodetector provides a signal whose intensity indicates the number of scintillation photons reaching that photodetector. The resulting signal, however, does not provide precise information on where the gamma ray photon interacted with the scintillating crystal. This imprecision can limit the spatial resolution of the resulting image.
One approach to enhancing spatial resolution is to allow scintillation photons to reach more than one detector. By observing the relative numbers of scintillation photons received by each detector, it is possible to determine the location at which the gamma ray photon interacted with the scintillation crystal.
The success of this approach depends in part on controlling the distribution of scintillation photons that reach the detectors. This spatial distribution of scintillation photons can be controlled by a optical element placed between the scintillating crystal and the detectors.